Bacterial pathogens must acquire iron to replicate and survive in mammalian hosts. The iron-porphyrin heme, bound to circulating hemoglobin, contains up to 80% of bodily iron. This fact has led to the hypothesis, which is backed by experimental evidence, that heme serves as a source of iron during infection. However, in the anthrax-causing bacteria B. anthracis, deletion of iron-regulated surface determinant (Isd) genes, which code for surface proteins that bind heme, did not reduce B. anthracis virulence in animal models of infection. These results suggest other factors contribute to iron uptake in this deadly pathogen. A hallmark of Isd systems is the presence of a conserved protein module termed the near-iron transporter (NEAT) domain, which mediates the transfer of heme into Gram-positive pathogenic bacteria. In silico analysis of the genome of B. anthracis indicates a non-Isd gene, designated BAS0520, is annotated to encode for a single NEAT-domain protein. The objective of this proposal is to determine if BAS0520 represents the missing link mediating heme uptake during anthrax infection. Specifically, we hypothesize BAS0520 is a surface protein that extracts heme from host hemoglobin, thereby promoting heme transport into the bacterial cell and enhancing iron-dependent replication in mammalian hosts. This hypothesis will be tested with two aims: 1. Determine the mechanistic function of BAS0520. Biochemical approaches will define the molecular and structural factors of the NEAT domain of BAS0520. 2. Determine the role of BAS0520 in iron acquisition and anthrax disease. Growth studies and animal infection models using fully virulent strains will be used to define which mechanisms of iron uptake are important for anthrax disease.